Applicants claim priority under 35 U.S.C. xc2xa7 119 of German Application No. 100 29 322.0 filed Jun. 20, 2000.
1. Field of the Invention
This invention relates to a method for finishing a valve seat for ball valves, in particular for fuel injection valves in internal combustion engines. In this invention, the workpiece to be finished and a finishing tool are preferably driven to rotate in an opposite direction. The axes of rotation of the workpiece and the finishing tool are aligned at an angle of from 1xc2x0 to 10xc2x0 in relation to each other, and whereby the finishing tool is applied to the entire circumference of the internal surface of the workpiece to be finished.
2. The Prior Art
A method with the feature described in part above is known from European patent EP 0 955 128 A2. The finishing tool consists of a cylindrical finishing stone. A seating surface with a circular shape viewed in the longitudinal section and extending circumferentially in the form of a ring, is worked into a conical valve seat by the known method. By means of the finishing treatment, it is possible to eliminate deviations in the shape measured in the cross section. Sealing systems with valve seats that have been finished according to the known method have a very good seat of the valve between the ball of the valve and the surface of the seat. However, when the cup-shaped surface of the seat is produced in the conical valve seat, a close inspection of the very short distances within which the ball of the valve is set in the course of the opening process shows that the through-flow characteristic of the valve changes. The gap between the cup-shaped seat surface and the ball of the valve, has a higher pressure loss than the gap between a conical surface and the ball of the valve of this gap, due to the short setting path of the valve ball. When the valve ball lifts off from the valve seat, this consequently results in a nonlinear characteristic, which is troublesome with very small valves, such as, for example, fuel injection valves. Another drawback is that when the ball valve opens, high adhesive forces acting between the valve ball and the cup-shaped surface of the valve seat have to be overcome.
The present invention provides a method of further developing the finishing treatment of the valve seat for ball valves so that the finished valve seat exhibits more favorable properties. A sealing system comprised of a valve ball and a finished valve seat is expected to open when low force is applied, and, when the ball lifts off from the valve seat, a large annular gap is immediately released, with low loss of pressure. Furthermore, it must be assured that the valve ball is seated on the valve seat as tightly as possible.
According to the method of the invention, the surface of the workpiece to be finished is provided with a crowned shape. Moreover, a finishing stone with a treatment surface that is resistant to wear is employed, to produce a narrow sealing zone with a convex contour in the longitudinal section, in the crowned surface of the workpiece, by partially removing some material. A finishing tool having a cone-shaped working surface is preferably used. Finishing the surface of the workpiece with a crowned shape thus permits the axis of rotation of the cone-shape finishing stone to be aligned at an angle in relation to the axis of rotation of the workpiece. The cone-shaped surface of the finishing tool applied to the inner surface of the workpiece is then finished over its entire circumference. By means of the finishing treatment as defined by the invention, a sealing zone is worked into the crowned surface of the workpiece that has a relatively narrow width, and is limited to the area where the ball and the seat of the valve are in direct contact. The width of the sealing zone usually amounts to a few tenths of one millimeter. Dimensional deviations from the circular shape originally present on the surface of the workpiece are nearly completely eliminated within the area of the sealing zone. Any dimensional deviations from the circular shape measured in the cross section can be reduced to less than 0.1 micrometer. The result is a sealing seat between the valve ball and the sealing zone that has a very low rate of leakage. Owing to the crowned, convex shape of the sealing zone, an annular gap with low loss of flow is released when the valve ball lifts off from the valve seat.
The valve seat to be finished may have a crowned surface of the sealing section, or may be provided with a crowned shape over its entire length measured in the direction of flow. In the embodiment first described above, the crowned surface of the workpiece to be finished forms an inwardly projecting section within a conical valve seat. In another embodiment, the crowned surface of the workpiece serving as the valve seat, limits the tapered space for receiving a valve ball, and connects two cylindrical sections of the workpiece that have different diameters.
The finishing stone or the workpiece is usefully inserted in a holding means that compensates for errors in the alignment between the workpiece and the finishing stone. The holding means is preferably a shaft made of hard rubber.
According to a further embodiment of the invention, a seating surface extending in the form of a circumferential ring and having a circular shape in the longitudinal section is worked into the already-finished sealing zone having a convex curvature. The sealing seat for a valve ball can thus be enhanced further in this way. However, as compared to the prior art of EP 0 955 128 A2, the cup-shaped deepening of the seating surface is substantially smaller because no dimensional deviations have to be compensated for as the deepening is produced in the form of a spherical indentation. An improved sealing seat can be obtained to that extent, without affecting the through-flow characteristic of the valve in any adverse way. In the finishing treatment of the sealing seat, the workpiece and a cylindrical finishing stone are driven in an opposite direction of rotation, whereby the axes of rotation of the workpiece and the finishing stone are aligned with each other at an angle of from 1xc2x0 to 10xc2x0, and the finishing stone rests against the entire circumference of the sealing zone. The area of contact of the cylindrical finishing stone on the surface side is usefully rounded in the form of a spherical indentation.